The production of volatile sulfur compounds such as hydrogen sulfide (H2S) during alcoholic fermentation is an issue that affects the brewing and winemaking industries. Hydrogen sulfide (H2S) is an undesirable by-product of the sulfate reduction pathway (FIG. 1). It is formed in Saccharomyces cerevisiae under fermentation conditions. Production of H2S by S. cerevisiae strains ranges from 0 ug/L to 290 ug/L, well above the human detection threshold of 11 ng/L (Amoore and Hautala 1983). Its undesirable quality stems from the fact that it introduces a rotten egg odor characteristic to wines and although H2S is a volatile compound and can be removed by aeration, it has the potential to form mercaptans and thiols which will persist in the wine due to the low pH (Thoukis 1962). Mercaptans and thiols present themselves as onion or canned vegetable aromas and where volatile H2S can be managed, removal of other undesired sulfur compounds is technically difficult and strips the wine of other flavor compounds.
The formation of hydrogen sulfide by Saccharomyces cerevisae is a well-documented problem in the wine, beer and sake industry (Acree et al. 1972, Eschenbruch et al. 1978, Giudici and Kunkee 1994, Jiranek et al. 1995, Rauhut and Kurbel 1994, Walker and Simpson 1993). Nutritional factors such as levels of nitrogen, vitamins and cofactors (Giudici and Kunkee 1994, Jiranek et al. 1995) and environmental factors such as temperature, pH, levels of elemental sulfur (Rauhut and Kurbel 1994), presence of sulfur dioxide (Stratford and Rose 1985) and levels of organic compounds containing sulfur (Acree et al. 1972) have been associated with the production of volatile sulfur compounds in fermented beverages. The differences in production of volatile sulfur compounds have also been attributed to differences in yeast strain metabolism (Acree et al. 1972, Spiropoulos et al. 2000).
There are at least six different classes of yeast strain behavior with respect to hydrogen sulfide formation: 1) elevated levels under all conditions; 2) low levels under all conditions; 3) elevated production below and above a threshold level of nitrogen; reduced production during a ‘window’ of nitrogen levels with sulfide increasing at nitrogen levels above or below this window; 4) elevated production in response to limiting micronutrient levels irrespective of nitrogen content; 5) elevated sulfide production only when limited for both nitrogen and micronutrients; and 6) elevated sulfide production with increased rate of fermentation, which may be related to fermentation rate and carbon dioxide evolution or to some other factor such as increased heat production (Spiropoulos 2000, Jiranek 1995, Giudici 1994, Linderholm 2006).
The existing method for stripping sulfides from wine is copper fining. Copper addition can lead to the catalysis of deleterious compositional changes as well as increase the amount of waste produced by wineries requiring special treatment, ultimately resulting in higher production costs for wineries and higher wine costs for the consumer. Further, use of copper as a fining agent may lead to high residual copper levels in wine. The Trade and Tax Bureau allows a residual copper level of 0.5 mg/L for wine (See, e.g., the worldwide website at regulations.justia.com/view/89060/). Winemakers who use copper to remove hydrogen sulfide must then take measures to reduce the copper levels in the wine. Given the adverse health effects associated with excessive copper ingestion, particularly neurological disorders such as Alzheimers, the World Health Organization has recommended dietary restrictions on consumption of this compound (See, the worldwide website at who.int/water_sanitation_health/dwq/chemicals/copper.pdf). The availability of commercial yeast strains unable to produce hydrogen sulfide or which produce reduced levels of hydrogen sulfide will eliminate the need for copper treatment of wines.
Thus, there is a need in the art for compositions and methods for reducing H2S levels in fermented beverages. The present invention meets these and other needs.